Method and system for controlling fluid flow in a fuel processing system

ABSTRACT

Fuel processing systems, which reform a hydrocarbon fuel to produce hydrogen suitable for use in a fuel cell, have multiple air inlets for various process steps. Controls and feedback loops are correspondingly complex. Controlling the multiple airflows to a pressurized fuel processor using a single onboard compressor and a number of low pressure drop valves is a significant challenge to overcome in the process of getting a reformed on board a vehicle. A method has been developed for controlling the compressor speed based on the airflow demand of the partial oxidation (POX) zone, without direct feedback from the other airflows in the system. This ensures that the principal zone of air consumption always gets the appropriate amount of air, thus controlling the temperature of that zone and the reaction chemistry effectively. This method also allows removal of extra flow sensors from airflows where the effect of changed airflow (e.g. temperature change) can be used as a feedback to an air controller instead of the actual airflow itself. Similar principles are applicable in the control of other flows, such as fuel and water, when several flows are fed by a common source.

RELATED APPLICATION(S)

This application claims the benefit of U.S. Provisional Application No.60/422,616, filed Oct. 30, 2002, the entire teachings of which areincorporated herein by reference.

BACKGROUND OF THE INVENTION

Fuel reformers and integrated fuel processors are well known forproduction of hydrogen. Historically, such fuel processors have beenused in large chemical plants, producing hydrogen for chemicalsynthesis. There is increasing interest in using such reactors for smallscale and/or mobile applications. In such uses, it is important tosimplify the control system as much as possible, to minimize both costand complexity, and to improve maintainability in a “consumer”environment.

In general, fuel reformers receive input flows of three fluids (i.e.fuel, air, and water), which undergo various reactions in the reformerto produce an output flow of hydrogen. In a first stage, for example,the fuel reformer catalyzes the reaction of a fuel with water to formhydrogen and carbon monoxide. This first step in the reaction isendothermic, and requires heat to be supplied to the catalytic reaction.This step is generally referred to as partial oxidation (POX) of thefuel, and is typically done by burning part of the fuel in the catalyticbed, either by combustion or by catalytic reaction. The catalyticversion of the POX reaction is often referred to as autothermalreforming (ATR).

As an alternative to the POX reaction, fuel and water can be reacted ina catalyst bed that is heated by a separate burner, which uses air andadditional fuel to create heat. This is known as “pure” steam reforming.

With both POX and “pure” steam reforming reactions, after the fuel andwater have undergone the initial catalytic reaction to produce hydrogenand carbon monoxide, subsequent steps are utilized to convert carbonmonoxide and water (as steam) into hydrogen and carbon dioxide. Thesesteps typically include the “water gas shift” (WGS) reaction, whichreacts carbon monoxide with water to produce carbon dioxide andhydrogen. As a final step, a preferential oxidation (PrOx) process isused to remove residual carbon monoxide using small amounts of air and acatalyst.

In an integrated fuel reformer/fuel cell system, the output hydrogenfrom the reformer is fed to a fuel cell, where it reacts with oxygen orair to produce electricity. The leftover hydrogen from the fuel cell isnormally burned with more air, and in some cases with additional fuel,to produce heat for the first fuel reforming reaction, or for preheatingfuel, air or steam. A fuel processor includes all of these reactions,including the use of leftover fuel cell gases.

Typically, each of the three primary inputs into the fuel processor(i.e. fuel, air, and water) is fed to more than one point of use. Usingair flow for an example, typically at least three separate inputs of airare required. These include air used to make heat for the reformingreaction; air for the PrOx reaction; air for the fuel cell; and air forthe terminal burner when present. In some cases, the leftover air flowfrom the fuel cell is sufficient to also support heat creation for thereforming process. Other configurations may require four or more airflows. Of these air inputs, the air flow for the fuel cell is often thelargest volume flow. However, the flow rate for the burner or ATR or POXreaction is often the most critical, because there must be a preciseamount of air provided to efficiently reform the fuel while maintainingtemperatures in safe limits.

In a small or mobile system, it is strongly preferred that only one aircompressor be used. (And in the case of water and fuel flow, that onlyone water or fuel pump be used.) For automatic control of the system, a“model” must be implemented on a system controller. An important methodof controller model design is performed by frequency-response analysis.The Nyquist stability criterion enables the investigation of both theabsolute and relative stabilities of linear closed-loop systems from theknowledge of their open-loop frequency response characteristics.Simplified system models are often employed to represent the controlsystem plant. An input-output model is a basic concept of a dynamicsystem interacting with its surroundings via input variables and outputvariables. An example of a single input, single output system could be apump-flow meter system. The input would be the pump command signal, andthe output would be the flow.

In the previously described system, the fuel processor has an airdelivery system with multiple airflows coupled to one source, theonboard compressor. The modeling of the air system therefore becomes adynamic system with multiple-inputs and multiple outputs. Theinput-output equations, even for relatively simple multi-input,multi-output models become extremely complicated. The conceptualsimplicity of using the input-output representation of a dynamic systemis lost in the complexity of the mathematical forms with models that arenonlinear, have many inputs and/or outputs, or simply are of an orderhigher than 3.

Accordingly, a control system for three or four coupled flows (forexample, the air inlet from the compressor and three independentoutlets) is surprisingly complex to implement, and prone to instability.It therefore typically requires direct measurement of each flow, whichis itself expensive. Similar considerations may also apply to water andfuel flows, depending on the details of system design. It would bedesirable to simplify the control of multiple flows of air, and of waterand fuel, in a fuel processor, both to minimize cost and to improvesystem stability.

SUMMARY OF THE INVENTION

This invention relates to simplified and improved methods for thecontrol of gas and liquid flows in a fuel processor which comprises afuel reformer, one or more hydrogen cleanup modules, and fluid flows toand from a fuel cell. In one embodiment, a method for simplifying thecontrol of flow of a fluid in a fuel processor comprises determining,from among a number of possible inputs for the fluid in the fuelprocessor, a first fluid input which requires the greatest precision ofcontrol of the rate of fluid flow. The rate of fluid flow at this firstinput is regulated based upon feedback from a sensor associated with thefirst fluid input, wherein such regulation occurs with a first timeconstant. The rate of fluid flow at each of the remaining inputs isregulated based upon feedback from at least one sensor so that the flowshave a regulatory time constant that is at least about three fold longerthan the time constant of regulation of the first flow and/or have aflow volume that is less than about 10% of the average flow volume ofthe fluid at the first input. In the case of air flow, for instance, thefluid input which requires the greatest degree of control is generallyinput to the POX unit of the fuel processor (or equivalently to the ATRunit, or to the burner supplying steam in the case of a steam reformingunit), in other words, air flow to the combustion that supplies heat forthe fuel reforming reaction.

BRIEF DESCRIPTION OF THE DRAWINGS

The foregoing and other objects, features and advantages of theinvention will be apparent from the following more particulardescription of preferred embodiments of the invention, as illustrated inthe accompanying drawings in which like reference characters refer tothe same parts throughout the different views. The drawings are notnecessarily to scale, emphasis instead being placed upon illustratingthe principles of the invention.

FIG. 1 is a schematic diagram of the flows of air and reformate in afuel processor and associated fuel cell; and

FIG. 2 is an outline of the control flows in the system processor.

DETAILED DESCRIPTION OF THE INVENTION

A description of preferred embodiments of the invention follows.

In this application, “fluid flow” refers to flows of any fluid in a fuelprocessor, including particularly air, water (as liquid and/or as steam)and fuel. “Fluid flow” also includes the flow of leftover hydrogen fromthe fuel cell, which is often recycled and used as an ancillary fuel inthe reformer. A “fuel processor” refers to a system comprising a fuelreformer, its associated hydrogen cleanup apparatus (usually WGSreaction and PrOx reaction), its ancillary equipment (compressors andthe like), and its connections with a fuel cell via flows of air, water,and hydrogen. A partial oxidation reformer (POX) includes the catalyticversion commonly called an ATR (autothermal reformer) unless otherwisestated. A “time constant” is the characteristic time for a response tobe completed to a defined extent, such as ½ or 1/e. A longer timeconstant produces a slower response. A response time may alternativelybe represented as the inverse of the time constant, (i.e. a“bandwidth”), where a larger value of bandwidth corresponds to a smallertime constant and to a faster response.

We have found that the control systems for fluid flows in a fuelprocessor can be greatly simplified by regulating the various flows withvalves or other controllers having different response times. To minimizecomplexity, the control methodology of the air system (or equivalently,the fuel or water system) is developed so that each flow into the systemis modeled and designed as an independent single input-output system.The logic diagrams for this controller are thus extremely simple.

In the case of air flow, for instance, the fluid input which requiresthe, greatest degree of control, typically the flow to the POX (orequivalently to the ATR unit), i.e. to the combustion that supplies heatfor the reforming reaction, is regulated with a short time constant(large bandwidth), and typically via direct feedback from a sensor, suchas an air flow sensor. Other types of sensors could be used, such as atemperature or pressure sensor, as an alterative to, or in combinationwith, the air flow sensor. The feedback from the sensor is used by thesystem controller to regulate the compressor. In one embodiment, this isperformed by having a variable rate compressor. Other types ofcompressors can be used, including compressors with variable pitch ofvanes, single-speed compressors with variable duty cycles, and otherknown types of compressors. Any form of compressor that can supply thefuel processor can be regulated as described herein. The total flow ofthe compressor is regulated by the controller using feedback from thePOX sensor or sensors. Then, to decouple the flows and simplify thecontrol system, the other air flows are controlled by controllers havingsubstantially longer time constants for response. For example, theresponse times of the other air flow controllers will typically be atleast about three times as long as the response time of the POX aircontroller, and preferably at least four times as long, and morepreferably at least five times to ten times (or more) as long. Largerratios of time constants (greater than about 10 times) will notsignificantly improve decoupling, but could be implemented if requiredfor other purposes. As will be described in more detail below, theeffect of adjustment of less-critical flows is seen by the controller asa variation in the more rapidly-controlled POX airflow. The rapidresponse of the POX flow control allows regulation of the other flows tobe decoupled from the POX flow, thereby greatly simplifying the controlalgorithms.

FIG. 1 shows a schematic of a typical POX-type reformer system 10. InFIG. 1, dotted lines represent air-flow related control lines to asystem controller 11. (Note that although a single system controller 11is shown here controlling all inputs of a particular fluid to thesystem, each fluid input into the system can have an associatedcontroller for receiving input data from the system—such as flow rates,temperatures, fuel input rates, etc.—and using this data to control theparticular flow rate, such as by adjusting a valve or varying compressorspeed.) Also shown in FIG. 1 is compressor 12, which in this example isa variable-rate compressor. Air from the compressor is fed to a plenum,and from the plenum to fuel processor components via illustratedcontrollable valves (V1, V2, V3, V4). Valve V1 feeds the initial fuelreforming unit, labeled POX. The POX unit has a flow sensor F1associated with air flow into V1. It may also or instead have anassociated temperature sensor T, or another sort of measuring device,depending on details of system design. The air flow rate, or othercontrol parameter, is communicated to the controller 11, which adjuststhe speed of the compressor 11 to maintain the air flow and/ortemperature of the POX unit within a selected range. (Note that in thisparticular embodiment, the valve V1 is entirely open in a normaloperating state, and is shut only in other system states.) The air flowcontrolling signal is filtered and processed to eliminate noise, andwill have a characteristic response rate R1, which may be expressed interms of bandwidth at the controller. (Note that a slower responsecorresponds to a smaller bandwidth, i.e., fewer possible cycles ofadjustment per second.) Alternatively, the feedback from the POX may bevia a temperature sensor T, or via a measurement of the influx rate offuel, since the required air flow rate is a proportion of the fuel inputrate. The proportion may vary depending on the system state—for example,startup vs. steady state—and the controller can be programmed to adjustthe proportion depending on the overall state of the system. In such asystem, the valve V1 is typically simply on or off, or, in someembodiments, the valve V1 may not be present in the system; in suchcases, flow rate is directly regulated via compressor speed. Theresponse rate R1 then refers to the response time of the sampling of theair flow, temperature, or other parameter, as used to regulate thevolume output of the compressor, for example by varying its speed.

Another control method having similar decoupling characteristics allowsvalve V1 to be a proportioning valve, of any convenient sort. In thiscase, a constant pressure is maintained in the plenum, and the POX airflow rate is controlled by the fraction of time that V1 is open, or thedegree to which V1 is open. The key sensed value could then be theplenum pressure, which could be sensed and controlled by adjusting thecompressor speed, or its volumetric output per unit time, or, with afixed speed compressor, its duty cycle. In each case, the input into thecontroller for controlling valve V1 will have a characteristic responserate or bandwidth, R1.

Valve V2 controls air flow into the PrOx (Preferential Oxidationreactor), which is part of the hydrogen cleanup system. As illustrated,the reformate leaves the POX unit and passes through the WGS (water gasshift) unit, where carbon monoxide is reacted with water to produceadditional hydrogen. The reformate then enters the PrOx unit to removeresidual carbon monoxide. The PrOx unit catalytically reacts residualcarbon monoxide with added air, to prevent fuel cell poisoning. Forefficiency, air usage in the PrOx unit should be minimized. The amountof air needed to remove residual carbon monoxide with the PrOx can bedetermined in any of several ways. For example, it can be calculated bythe controller based on the rate of fuel input, as adjusted for thesystem state. Alternative inputs to the controller include PrOxtemperature, and values from a carbon monoxide sensor.

As with the controls for V1, the inputs supplying data for controllingV2, or the control systems acting on the data, will also have acharacteristic response rate, R2. The PrOx is illustrated here as havingone air inlet, but in practice there may be several air inlets to aPrOx. These are not illustrated; they may be controlled by furthervalves from a plenum, either the one illustrated or a separate plenumdownstream of V2; or may be proportioning orifices in a second plenum,or otherwise arranged.

Valve V3 controls air flow into the fuel cell. In the arrangementillustrated, the fuel cell air is then used as the sole or primary airsource for an auxiliary burner or “tail gas combustor” (TGC). However,the TGC could instead or in addition have a separately regulated airsupply (V4). Air flow through fuel cell inlet valve V3 can be regulatedaccording to one or more of several variables, including fuel inputrate, electricity production rate or demand rate, or other measurable orcalculable parameters. The input into the controller for controllingvalve V3 will have a characteristic response rate or bandwidth R3, andV4, if present, a bandwidth or time constant R4.

The design shown in FIG. 1 is close to the minimal number of requiredair inlets into the integrated system (noting that V4 is optional insome systems). Any additional air flow control valves V5, etc. that maybe present due to details of system design will likewise have responserates R5, etc.

In the system illustrated in FIG. 1, the rate of air flow to thereforming element of the system, here labeled as POX, is the flow raterequiring the most precise degree of control, relative to the rate ofair flow to the other system components. This is because the reformermust be operated at a high temperature (typically in the range of 700deg. C. or above; lower with methanol fuel), and the operatingtemperature must be controlled to be within a relatively narrowrange—high enough to provide heat for the reforming reaction at a ratesufficient to reform the non-oxidized fuel, but low enough not to damagesystem components, including the catalysts and structural elements.Moreover, combustion of fuel in excess of that required to reform therest of the fuel is wasteful and reduces system efficiency.

The other air flows, in this particular embodiment, are less critical,and do not need to be regulated as tightly. The PrOx supply isrelatively low in volume compared to the reformer heating air flow, andso a less tight regulation may be acceptable. Moreover, the volume ofthe PrOx flow is less than 10% of the reformate flow, and more typicallyless than 3% of the POX flow. Therefore, regulation of the PrOx flow atany response rate will not significantly perturb the system pressure orthe flow rate into the POX.

The third flow is the air supplied to the fuel cell and/or the TGC,regulated by V3 and/or V4. These flows are large in volume, but theexact amount is not as critical as the flow of POX air in terms ofregulation, since air is normally supplied in excess to both the fuelcell cathode and the TGC or equivalent. The associated response timeconstants are R3 and R4.

These considerations allow a great simplification of the controlalgorithm. The response rate R1 of the POX control is selected to be thefastest response rate in the air control system. The other responserates R2, R3, etc., are selected to be slower than the response rate R1.This typically requires that the response rates R2, R3 of the othercomponents be at least about a factor of about three or four timesslower than the rate for the POX, or more preferably a factor of atleast about.,5 times, or at least about 10 times slower. (As notedabove, values above 10 are possible in the invention, but are largerthan is required for stability and decoupling.)

However, when another flow is small enough to not perturb the pressurein the manifold, or equivalent structure, then the regulation of theflow in that component may be at any response rate. For example, asnoted, this criterion will often be applicable to the PrOx flow.

An example of the logic flow of such regulation is shown in FIG. 2 (withreference to the system components of FIG. 1). The POX flow, in thiscase regulated by a flow meter (F1), is measured in the POX controllerand compared to a set point with a relatively rapid response time (0.2Hz bandwidth). Based upon the measured flow rate, the POX controllerdirectly controls the compressor 12 to provide the desired POX flowrate.

The TGC/fuel cell flow, which is sufficiently large so that variationsin its flow rate will significantly perturb overall system pressure, isregulated via fuel cell demand and/or TGC temperature with a slowercontroller response, here 0.05 Hz bandwidth. Here, the TGC/fuel cellcontroller does not directly control the compressor, but instead onlyregulates the inlet valve or valves associated with the TGC/fuel cellcomponents (i.e. V3 and V4 in FIG. 1).

Similarly, the small PrOx air flow is regulated via its control valve(V2), and not via regulation of the compressor. Because the PrOx flow issmall, specifying the response time is not required, because PrOx flowwill not perturb system pressure enough to cause oscillations or otherinstability. However, it is convenient to have a slower response time R2in the PrOx controller. A key aspect of the three flows illustrated isthat they do not need to be implemented as coupled flows. Because of thedecoupling provided by the difference in time constants, no coupling isrequired in the computation.

In practice, the POX air is regulated rapidly compared to the other airflows. The slower variations in the flows to the TGC (and to the fuelcell if on the same compressor) function as a slowly varying backgroundto the POX controller. The POX controller rapidly corrects thecompressor flow to maintain the POX flow rate, and thus the overall airpressure is quasi-constant even while the TGC flow is being adjusted.Because there is only one signal to the compressor, the influence of thevarious valve settings on the manifold pressure does not need to becomputed. This greatly simplifies the creation of a control algorithmfor the system, saving expense and increasing reliability. As a furtherbenefit, it is much easier to adjust settings of individual airflows inresponse to overall system state, since there is still only one input toeach control element. An additional benefit is that most or all of thecontrol loops can use sensors other than air flow sensors. Only the POXair flow rate is a likely candidate for use of an air flow sensor in itsregulation. Minimization of use of air flow sensors is important forcost reduction, because at low pressure drops, as often encountered inthese systems, the required sensors are relatively expensive compared tomonitoring temperature, or fuel injection rate.

This system has been described in terms of a POX reformer, whichincludes the catalytic ATR (autothermal reforming) variant. The systemis also applicable to a “pure steam reformer” system. In such a system,a separate air supply and fuel supply are fed to a burner that is inthermal communication with a catalytic reforming zone, and only fuel andsteam enter the actual reforming zone. The control considerations areessentially identical, with rapid control of the burner air required(similar to the control of the POX described above), and slower responsetime control of the fuel cell air, the PrOx air, and TGC air ifseparately supplied.

Likewise, the topology is illustrated here by having three or moreseparate inlets drawing from a common manifold. However, one or more ofthe PrOx, fuel cell, or TGC inlets could depend from the airflow beingdirected to the POX, achieving the same effect in terms of controlsimplification.

Fuel and water flows are typically less branched, but similar methodscan be used to decouple branches of these flows as well, when requiredto prevent instability. For example, in a fuel processor, fuel issometimes supplied both to the reforming zone and to an auxiliaryburner, and the latter flow is influenced by the amount of hydrogenreturning from the fuel cell. The burner flow is in this case typicallysmaller, and also typically less critical, and it can be decoupled fromthe main flow by use of a slower control loop, thereby decoupling theflows and making it unnecessary to consider the burner flow whenadjusting the fuel pump to supply the reformer. In a steam reformer,fuel flows to both the reforming zone and to the integrated burner thatheats the reformer are similar in magnitude. If they are to be suppliedby a common pump or other regulator, then the flow of one—for example,the burner fuel—can be regulated with a faster time constant than theother—for example, the reformer fuel supply. This decouples the flowsand prevents oscillations. (Which of these flows is the most criticalwill depend on details of system design.)

Water is used in the fuel processor to make steam, and the steam may insome systems be injected into the reforming section at two separatelocations in similar quantities (to the reformer itself, and to thewater-gas-shift unit). If the steam flows, or water flows leading tosteam formation, are separately regulated (as opposed to simply beingproportioned), then the regulating valves should likewise have differentresponse times to decouple the flows. Water is also used in severalother locations in the fuel processor system, including uses for coolingof reformate and of the fuel cell. When it is possible to supply theseuses with a common pump, similar control considerations apply.

While this invention has been particularly shown and described withreferences to preferred embodiments thereof, it will be understood bythose skilled in the art that various changes in form and details may bemade therein without departing from the scope of the inventionencompassed by the appended claims.

1. A method for simplifying the control of flow of a fluid in a fuelprocessor, the method comprising the steps of: determining, from among aplurality of inputs for the fluid in the fuel processor, a first fluidinput which requires the greatest precision of control of the rate offluid flow; regulating the rate of fluid flow at the first input basedupon feedback from a sensor associated with the first fluid input,wherein such regulation occurs with a first time constant; andregulating the rate of fluid flow at each of the remaining inputs basedupon feedback from at least one sensor so that the flows satisfy atleast one criterion selected from: i) having a regulatory time constantthat is at least about three times greater than the time constant ofregulation of the first flow; and ii) having a flow volume that is lessthan about 10% of the average flow volume of the fluid at the firstinput.
 2. The method of claim 1 wherein the fluid comprises air, and therate of fluid flow at the first input is regulated by controlling acompressor coupled to the first input.
 3. The method of claim 2 whereinthe first input flow comprises air for providing heat for a fuelreforming reaction.
 4. The method of claim 2 wherein the first inputflow comprises air for a combustor or burner that supplies the heatrequired to reform fuel in a fuel reformer selected from a partialoxidation reformer (POX), an autothermal reformer (ATR), and a “pure”steam reformer.
 5. The method of claim 1 where the fluid comprises agaseous or liquid fuel, and the input is supplied by one of a fuelcompressor and a fuel pump.
 6. The method of claim 1 wherein the fluidcomprises liquid or gaseous water.
 7. The method of claim 1 wherein theregulatory time constant for the remaining inputs is at least about fivetimes greater that the time constant of the first input.
 8. The methodof claim 1 wherein the regulatory time constant for the remaining inputsis at least about ten times greater that the time constant of the firstinput.
 9. The method of claim 1, wherein the flows of at least one fluidcan be entered into a control algorithm without requiring coupling ofthe flows to each other in the computations required to control thesystem.
 10. A fuel processor comprising: a fuel reforming unit having afluid inlet for varying the rate of input of a fluid; a hydrogen-cleanupunit having a fluid inlet for varying the rate of input of the fluid; afluid conduit for providing the fluid to a fuel cell, the fluid conduithaving a fluid inlet for varying the rate of input of the fluid; acontrol system which determines, from among the fluid inlets of fuelreforming unit, the hydrogen-cleanup unit, and the fluid conduit for thefuel cell, a first fluid inlet which requires the greatest precision ofcontrol of the rate of input of the fluid, the control system regulatingthe rate of fluid flow at the first fluid inlet based upon feedback froma sensor associated with the first fluid inlet, wherein such regulationoccurs with a first time constant, the control system further regulatingthe rate of fluid flow at each of the remaining fluid inlets based uponfeedback from at least one sensor so that the flows satisfy at least onecriterion selected from: i) having a regulatory time constant that is atleast about three times greater than the time constant of regulation ofthe first inlet; and ii) having a flow volume that is less than about10% of the average flow volume of the fluid at the first inlet.
 11. Thefuel processor of claim 10, wherein the sensor associated with the firstfluid inlet comprises a fluid flow rate sensor.
 12. The fuel processorof claim 10 wherein the fluid is air.
 13. The fuel processor of claim 12wherein the control system varies the rate of fluid flow at the firstinlet by controlling a compressor coupled to the first inlet.
 14. Thefuel processor of claim 13 wherein the air from the compressor is fed toa plenum, and from the plenum to a plurality of fuel processorcomponents via at least one controllable valve.
 15. The fuel processorof claim 14 wherein the first fluid inlet comprises an inlet to the fuelreforming unit.
 16. The fuel processor of claim 15 wherein the rate ofinput of fluid to the fuel reforming unit is controlled by varying theoutput of the compressor.
 17. The fuel processor of claim 16 wherein therate of input of fluid to the hydrogen-cleanup unit and the fuel cell iscontrolled by adjusting valves associated with the hydrogen-cleanup unitand the fuel cell.
 18. The fuel processor of claim 10 furthercomprising: a tail gas combustor having a fluid inlet for varying therate of input of a fluid, wherein the control system regulates the rateof fluid flow to the fuel reforming unit, the hydrogen-cleanup unit, thefuel cell, and the tail gas combustor.
 19. The fuel processor of claim10 wherein the fuel reforming unit comprises a partial oxidationreformer.
 20. The fuel processor of claim 10 wherein the fuel reformingunit comprises an autothermal reformer.
 21. The fuel processor of claim10 wherein the fuel reforming unit comprises a pure steam reformer. 22.The fuel processor of claim 10 wherein the hydrogen-cleanup unitcomprises at least one of a water gas shift reactor, and a preferentialoxidation reactor.
 23. The fuel processor of claim 10 wherein the fluidcomprises water.
 24. The fuel processor of claim 10 wherein the fluidcomprises fuel.
 25. The fuel processor of claim 10 wherein the controlsystem varies the rate of fluid flow at the first inlet by controlling apump coupled to the first inlet.